Modern medicine uses many procedures where electrical signals are received from or delivered to a mammalian patient's body through skin. The interface between medical equipment used in these procedures and the skin of the patient is usually some sort of biomedical electrode. Such biomedical electrodes typically include a conductor which must be connected electrically to the equipment, and a conductive medium adhered to or otherwise contacting mammalian skin.
Because all biomedical electrodes are based on the transmission of electrical signals to and from mammalian skin, good electrical conductivity is required, as free as possible from electrical impedance during a diagnostic, therapeutic, or electrosurgical procedure. Electrical impedance is disruptive to diagnostic or therapeutic procedures where low voltages or low currents are transmitted to or received from a patient's body. Lowering electrical impedance for biomedical defibrillation electrodes can lower the possibility of a less than effective electrical circuit at a moment of criticality for live-saving measures. Further, in the case of dispersive electrodes, an increase in impedance at one part of the contacting surface can increase the current density in places where the impedance is relatively lower. Electrical burns are a known problem in the use of dispersive electrodes when the current density in a region of the electrode becomes too high. Biocompatible, hydrophilic pressure-sensitive adhesive compositions (hereafter "PSA hydrogels") can be employed as the conductive medium for biomedical electrodes because such PSA hydrogels have excellent adhesion to a variety of types of mammalian skin, and good mechanical strength, and, with inclusion of an electrolyte, good electrical conductivity. Examples of PSA hydrogels based on copolymers plasticized with water and humectant that are preferred for biomedical electrodes are described in U.S. Pat. Nos. 4,524,087; 4,539,996; 4,554,924; and 4,848,353 (all Engel), which also describe constructions of biomedical electrodes suitable for use as dispersive biomedical electrodes. Also, EPO Publication 0 322 098 (Duan) describes another PSA hydrogel based on plasticized poly(N-vinyl lactam) which can be used as the conductive medium for biomedical electrodes. The disclosures of all of the co-assigned Engel patents and Duan European Patent Publication are incorporated by reference herein.
Mechanical and electrical properties of a PSA hydrogel useful in a biomedical electrode are dependent on the content of water or moisture in the PSA hydrogel as formed and then as stored and used. In each of the Engel patents and the Duan publication, water is employed in the formation of the PSA hydrogel and water is required for electrical conductivity by interaction with an electrolyte. These PSA hydrogels are one phase compositions which are both hypoallergenic pressure sensitive adhesives and ionically-conductive media for transmission or reception of electrical signals.
By contrast, U.S. Pat. No. 4,588,762 (Mruk et al.) describes a heterogenous, pressure-sensitive, electrically conductive adhesive for disposable biomedical electrodes, consisting of two phases: a viscoelastic polymeric adhesive phase and an electrically conductive aqueous phase containing a water receptive polymer, a humectant, and an electrolyte. The humectant is present in the aqueous composition phase to minimize evaporation of water or moisture present during formation of the adhesive from the aqueous zones extending through the thickness of the composition. However, there is no electrical conductivity in the viscoelastic polymeric adhesive phase. Further, a two phase pressure-sensitive system described by Mruk et al., having regions without electrical conductivity is otherwise unsuitable for dispersive biomedical electrodes.
European Patent Publication 0 085 327 (Cahalan et al.) describes electrically conductive compositions useful for biomedical electrodes wherein the conductive composition comprises an interpenetrating polymer network consisting essentially of a hydrophilic crosslinked polymer formed from a water soluble monomer (such as an N-sulfohydrocarbon-substituted acrylamide, hydroxymethylmethacrylate, and potassium acrylate polymers) and a hydrophilic polymer which is not crosslinked (such as polyacrylic acid, polyvinylpyrrolidone, and non-crosslinked N-sulfohydrocarbon-substituted acrylamides), a humectant, and water.
Incorporation of water swellable polymers into other materials has been used for bandage and wound dressing type products.
European Patent Publication 0 297 769 (Cilento) discloses a pressure sensitive acrylic adhesive mass which is made hydrophilic by blending one or more water moisture absorbing, water moisture transmitting substances into the acrylic mass. These substances can be water soluble or swellable hydrocolloids (such as cellulosics, gums and the like) or super absorbents (such as substantially water insoluble, starch-acylonitrile graft copolymers, water insoluble, cross-linked sodium carboxymethylcellulose, or water insoluble, crosslinked dextran). European Patent Publication 0 107 376 (Thompson et al.) discloses polypyrrolidone gel dressings prepared by dissolving between 15% and 25% by weight of polyvinylpyrrolidone in water and cross-linking the polypyrrolidone by means of ionizing radiation. Various reinforcing materials, such as nylon gauze, cellulose, reticulated polyethylene or polypropylene, can be included as reinforcing agents.
U.S. Pat. No. 4,477,325 (Osburn) discloses a skin barrier composition of a ethylene and vinyl acetate copolymer (EVA) resin, at least one water absorbing particulate hydrocolloid polymer, and a water-insoluble, dry tack-providing elastomer such polyisobutylene, to be useful as an ostomy appliance and the like. After mixing and molding, the composition is subjected to ionizing irradiation to form cross-linked polymer networks.